1. Field of the Invention
The present invention relates to a particulate detecting sensor for detecting particulate such as dust contained in smoke generated from a fire or in air. More particularly, this invention is concerned with a photoelectric particulate detecting sensor for detecting the presence of particulate by detecting scattered light occurring due to the particulate.
2. Description of the Related Art
In general, a particulate detecting sensor such as a high-sensitivity smoke sensor or dust monitor irradiates a light beam from a light source to a detection area, detects scattered light occurring due to particulates existent in the detection area, and thus detects the presence of the particulates. A light-emitting diode (LED), semiconductor laser (LD), xenon lamp, or the like is used as the light source. Scattered light occurring due to particulates is so weak that it is necessary for reliable detection of particulate to use a light source capable of supplying a large quantity of light.
However, many light sources capable of supplying a large quantity of light are generally characterized by a large quantity of heat generation and a great temperature dependence. There is a possibility that when temperature rises, a desired light-emission characteristic cannot be exerted.
A light beam emitted from a light source is spread conically or converged on a spot, and then projects into a detection area. Using a conically-spread light beam, a wide detection area can be set. However, since the light is radiated widely, it is reflected on the surface of an inner wall or the like in the detection area. Even when no particulate exists in the detection area, light referred to as stray light that is not scattered light is therefore readily detected by a light receiver. For constructing an optical system having a high signal-to-noise ratio, a large space is required for processing stray light. This results in enlarging the size of the particulate detecting sensor. On the other hand, when the spot convergence is adopted, a detection area is minimized although an optical system offering a high signal-to-noise ratio can be constructed. This technique is therefore unsuitable for particulate detection of volumes of air.
As mentioned above, as far as the known particulate detecting sensor is concerned, when a conically-spread light beam is used, there arises a problem that a high signal-to-noise ratio cannot be attained unless the entire sensor is made large in size. When a light beam converged on a spot is used, there arises a problem that it becomes difficult to detect particulate in volumes of air.
In the known particulate detecting sensor, there is another problem that a desired light-emission characteristic cannot be attained because of a temperature rise occurring when a light source is driven, and eventually it becomes impossible to detect particulate reliably.
The present invention attempts to solve the foregoing problems. An object of the present invention is to provide a particulate detecting sensor capable of reliably detecting particulate in volumes of air at a high signal-to-noise ratio despite having a compact design.